PITX2 Expression in Tooth Initiation and Replacement in the Gecko

Objectives: Tooth development is often studied in the mouse and yet the dentition is highly specialized - the incisor continuously erupts and molars are not replaced. Our goal is to understand the biological basis of tooth replacement in a suitable animal model, Eublepharis macularius, which replaces its teeth throughout life. The objective is to identify odontogenic potential in the gecko dental epithelium by mapping expression of PITX2 (paired like homeodomain 2), the first transcription factor expressed in the mammalian dental epithelium. Moreover, mutations in PITX2 cause Axenfeld-Rieger syndrome which features oligodontia.
Methods: A growth series of geckos, including embryos, juveniles, and adults were examined (n=10). Immunofluorescence staining of histological sections was carried out with a polyclonal antibody to PITX2 (1:500 dilution, R&D systems).
Results: PITX2 is exclusively expressed in epithelium of the teeth and not in other oral epithelia. In embryos, the greatest proportion of labeled cells was in the successional laminae (77% of cells were positive). The dental lamina had 35% of the cells labeled. The enamel organs were also positive for PITX2. In contrast, in adults, PITX2 expression is largely absent from the aboral dental lamina (9% of cells labeled). Closer to the teeth there is signal in the adjacent dental lamina (50% labeled cells) as well as between the generational teeth (60% labeled cells). The enamel organ and successional lamina are also strongly positive, similar to the embryo
Conclusions: PITX2 is restricted to epithelia either in the tooth bud itself or areas of epithelia with odontogenic potential, especially the successional lamina. The PITX2- positive cells in the adult correlate strongly with areas that form teeth. The quiescent dental lamina close to the oral epithelium is largely negative. It is very interesting that PITX2 in strongly expressed between the generations of teeth in the dental lamina. This pattern can explain the presence of supernumerary teeth in humans. Indeed, a precocious downregulation of PITX2 in the successional lamina could be linked to permanent tooth agenesis. Our work validates the gecko as a model to study tooth development since the major transcription factor required for tooth initiation is conserved with humans.